Geology Question

I recently made a trip to and from Raleigh NC and I happened to pass Pilot Mountain. It is an odd little mountain raising a bit away from most of the others, but the strangest part is it looks like a cylinder of rock is pushed up straight from the middle.

I understant about the plates smashing together and forming mountain ranges, but how did this single “tube” of rock get pushed straight up?

It didn’t get “pushed up.” It eroded down.

http://en.wikipedia.org/wiki/Pilot_Mountain_%28North_Carolina%29

http://en.wikipedia.org/wiki/Monadnock

That makes some sense… thanks. I don’t pretend to know much of anything on the subject, so what makes this center “tube” erosion proof? Will it continue to erode until eventually there will just be the column of granite or whatever?

The erosion rate of the rock in the center is slower than the rock surrounding it, as stated in the quote I offered.

It is protected by a layer of harder rock on top of it. Just take a look at the pictures here and you’ll see what I mean. Not knowing anything about the geology of the place I’d guess that the light rock is sandstone or some other softer sedimentary rock type that at one time has been covered with lava at a volcanic eruption.

Usually, inselbergs like that form when there’s a cap of resistant rock, but it could also be structural (the surrounding area was more faulted/jointed and easier to erode. If that mountain was here, I’d suspect a lava or sandstone cap on something softer (those kind of isolated koppies are very common in the local geomorphology), but sometimes it’s just the breaks of the particular regional erosion scheme. The nearby mountain range kind of confirms this genesis.

Floater, I’d hesitate to call sandstone a “softer” sedimentary rock. Especially when it’s metamorphosed into quartzite, like Pilot Mountain, it will be very resistant to general weathering.

The best known example of this is Devil’s Tower, in Wyoming, which is a volcanic plug that is left after the softer material around it has eroded away.

I meant, of course, softer than the layer on top of it.

Actually, there’s some debate as to its actual origin.

Yeah, that “more resistant igneous rocks” idea is a bunch of nonsense. Everyone knows it was formed by a giant bear.

Well, those seven Indian girls shouldn’t have teased him.

Thanks guys… and I appologize that you have to teach me this stuff at a kindergarten level. I still don’t understand how this perfect circle of “hard rock” ended up in the exact center of a mountain. It makes sense for the volcano examples, but was this once a volcano? Seriously I have no clue but I’ve never thought of this area as a hot bed for volcanos.

Anyway, I’m sure I’ll have a conversation similar to this with my buddy who drove there and back and looked at it with me…

Me: I found out it didn’t push up, instead the rest washed away from it.
Him: I thought the question was if Pilot Mountain was the source of Mount Pilot on Andy Griffith.
Me: If you look at it from this angle it looks like a giant nipple.

Yes, there are some things way out of my vast area of expertise, and this is definitely one of them.

Think of it in reverse.

Originally, there was a giant layer of hard rock. This layer mostly eroded away at the edges, as softer rock below it was under cut. But the part of the layer in the center was not eroded as quickly and protected the softer rock in the center. As the outsides continue to erode, you naturally wind up with a mountain that has a roughly circular cap in the center.

So… The mountain exists because the harder rock protected that little bit of softer rock underneath. It’s circular just because it eroded from the outside in.

If more of the rock was left, you might have a mesa, which is not nearly as circular and much larger. Get bigger still and you get something like a big chunk of central Idaho, where the layer of hard rock is still mostly intact and you only realize its there when you see how the Snake river has cut through it and the lower layers.

You have a great career as a kindergarten teacher in your future… Thank you. That actually makes sense to me.

This is what you get if it doesn’t erode to a circle. Pretty, isn’t it?